The present invention generally relates to a fuel control system for an internal combustion engine and, more particularly, to a closed loop fuel control system for controlling the supply of a mixture of air with combined fuel to the engine in dependence on the composition of a particular component of exhaust geses emitted from the engine.
The term "combined fuel" hereinabove and hereinafter referred to for the purpose of the present invention is to be understood as meaning a mixture of two or more ignitable vaporizable fuels such as gasoline, kerosene, and aliphatic alcohol including, for example, ethanol and methanol.
With the rise of the oil price in recent years, attempts have been made in some countries in the world to use, for the combustion in an automobile internal combustion engine, of an alternative fuel, i.e., combined fuel, particularly consisting of gasoline mixed with alcohol, for example, ethanol or methanol, in a predetermined mixing ratio, for instance, in an amount within the range of 10 to 30% relative to the total weight of the combined fuel. The use of such a combined fuel, in contrast to a single fuel of gasoline now largely used, appears to increase in the near future and experiments are currently carried out to evaluate it.
In the course of the experiments, it has been found that gasoline and alcohol comprising the combined fuel tend to separate from each other while they are accommodated in a mixed state in a vehicle fuel tank. In addition, it appears that, when the time comes for the combined fuel to be available from local gas stations where gasoline and alcohol are mixed together, the mixing ratio of alcohol relative to gasoline would possibly vary from one gas station to another. In either case, the concentration of the alcohol in the combined fuel, and, hence, the ratio of air to combined fuel in an air-fuel mixture being supplied to the engine cannot be fixed for a given operating condition of the engine.
As is well known to those skilled in the art, the higher the concentration of the alcohol in the combined fuel, the lower the stoichiometric air-fuel mixing ratio. Therefore, where the concentration of the alcohol varies by the reason stated hereinabove, it is necessary to adjust from time to time the air-fuel mixing ratio (i.e., the ratio of air to combined fuel,) to the stoichiometric value in dependence on the change of the alcohol concentration so that the combustion of an air-fuel mixture, i.e., a mixture of air with combined fule, can take place efficiently in the engine.
This adjustment can automatically accomplished by the use of any known closed loop fuel control system wherein an electrical signal generated from a composition sensor for detecting the concentration of a particular component, for example, oxygen, contained in exhaust gases from the engine, which concentration is a function of the air-fuel mixture supplied to the engine, is fed back to a fuel supply system to control the supply of fuel to be subsequently mixed with air, thereby adjusting the air-fuel mixing ratio of an air-fuel mixture to be subsequently supplied to the engine to the stoichiometric value. An example of this known fuel control system is disclosed in the U.S. Pat. No. 4,186,691.
Although the accurate adjustment of the air-fuel mixing ratio of the mixture of air with combined fuel can successfully be accomplished by adopting the known fuel control system despite that the latter has obviously been developed for use with the mixture of air with gasoline, this does not mean that the combustion of the mixture of air with combined fuel as well takes place efficiently in the engine in terms of the engine power output. This is not only because alcohol has a property of low speed in combustion than gasoline and the combined fuel, i.e., the mixture of gasoline with alcohol is accordingly much slower to ignite and burn than the single gasoline, but also alcohol tends to evolve a larger amount of latent heat upon vaporization than gasoline does, More specifically, where the concentration of alcohol in the combined fuel or in the air-fuel mixture is relatively low, an engine knocking is apt to occur during a low speed high load operating condition of the engine and it is necessary, in order to avoid this tendency, to delay the ignition timing somewhat from the ignition timing at which the MBT (Minimum Advance for Best Torque) can be achieved with the combustion of the air-gasoline mixture. On the other hand, where the concentration of the alcohol is relatively high and if the ignition timing has been so set as hereinabove described for the purpose of avoiding the engine knocking, the ignition timing would be late for the combustion of the air-fuel mixture with the result of the reduced combustion efficiency because of the above described nature of the alcohol relative to the gasoline.
The above described problems may possibly be solved by providing a similar closed loop fuel control system, substantially in parallel with the previously discussed fuel control system, solely for controlling the ratio of mixture of gasoline with alcohol to a predetermined value by regulating the supply of alcohol or gasoline to be mixed with gasoline or alcohol, respectively, prior to the preparation of the combined fuel. However, this concept requires the use of not only the two, seemingly similar control systems but also separate fuel tanks and their associated fluid supply line, and is therefore neither economical nor payable for the cost-effectiveness.